Physical activity levels decline dramatically during adolescence, particularly among girls. Given the well-known health and psychosocial benefits of regular physical activity, developing innovative interventions to encourage adolescent girls to adopt and maintain a physically active lifestyle is desperately needed. Interventions that cultivate partnerships among schools, families, and communities to teach lifestyle-oriented physical activity skills, provide supportive environments, and enhance opportunities to engage in physical activity are likely to be met with greater success than single-component interventions. TAAG is a multi-center randomized field trial developed in response to this concern. Thirty communities with middle schools and community resources across five geographical locations will constitute the trial sample. Approximately 4,000 girls in middle school will provide data to test the effectiveness of the intervention on the primary outcomes of the study, daily energy expenditure and cardiorespiratory fitness. We will conduct the trial in six school- community partnerships in Baltimore City and Baltimore County. Formative research will be conducted to determine beliefs about middle school girls' physical activity and sedentary habits among influential peers, adults, and community agencies. The school intervention will be conducted in randomized intervention schools in 7th and 8th grade PE and Health Education classes. We propose basing the intervention on the AAHPERD Physical Best Program, and incorporating a family support program. Community agencies associated with the intervention schools will partner by assisting with before- and after-school programs and providing additional programming attractive to adolescent girls. Girls will be followed for primary and secondary outcomes at the end of 7th and 8th grade to determine the effectiveness of the intervention. Additional follow-up in 9th grade will determine whether the intervention was successful in maintaining effects. The main analysis will compare the intervention versus the comparison communities for overall daily energy expenditure and cardiorespiratory fitness. Secondary outcomes are the effects of the intervention on patterns of physical activity and sedentary behaviors; body composition; initiation of smoking; and self-esteem. Additional analyses will evaluate self-efficacy, social support, perceived stress, school performance, and community involvement. Results will determine the ability of a comprehensive school and community-linked intervention to reduce the decline of physical activity and cardiorespiratory fitness typically observed among adolescent girls.